This invention relates to systems, apparatus and methods for drying material in a drying chamber, such as a hopper through which solid bulk material to be dried is passed. The invention is more particularly concerned with systems, apparatus, and methods for reducing the moisture content of solid particulate or pelletized material, such as plastic pellets supplied to industrial molding and extrusion machinery, food products, animal feed, chemicals or pharmaceuticals.
Conventional systems for drying particulate materials such as plastic pellets have relied upon the use of desiccants to remove moisture from a stream of drying air passed through a bed of the material. The desiccant, which is typically a molecular-sieve type material (e.g., zeolite), captures moisture from the air stream to produce very low dew point air which is in turn supplied to the material bed to dry the material to a desired moisture content level. In a typical system, the desiccant is situated in a unit disposed downstream from the particulate bed in a closed loop, and the dehumidified air from the desiccant unit is recirculated to the bed by a blower. A heater situated between the desiccant unit and the material bed heats the low dew point air to a desired drying temperature for supply to the bed.
The recommended dew point of air for drying plastic pellets is ordinarily below 0xc2x0 F., and typically in a range of about xe2x88x9220xc2x0 F. to about xe2x88x9250xc2x0 F. (or lower). Desiccant type drying systems can readily provide such low dew point air and have become quite popular over the years.
Notwithstanding their popularity, desiccant type drying systems have significant drawbacks. These arise primarily from the fact that desiccant materials must be regenerated periodically in order to maintain their effectiveness. Desiccants dehumidify by adsorption. When used over a period of time, a desiccant material will become loaded with water and lose its effectiveness as a drying medium. To restore its effectiveness, the desiccant material is regenerated from time to time, usually by flowing a heated air stream through the desiccant unit to drive off the adsorbed moisture. This requires that the desiccant unit be taken off-line, interrupting the drying process, or that the drying system include a second desiccant unit used alternately with the first desiccant unit, or which is operated such that its on-line time at least overlaps the regeneration cycle of the first unit.
In systems using a single desiccant unit, the down time associated with the desiccant regeneration cycle results in reduced material throughput. Systems employing multiple desiccant units can avoid this problem, but they are more expensive due to the need to provide additional desiccant units and correspondingly more complex system controls.
The present invention avoids the drawbacks of conventional desiccant type drying systems.
In a preferred embodiment, a system of the invention has two sub-systems, a first of which includes a dryer and a heater to supply dried and heated gas to a first portion of a drying chamber, and a second of which mixes gas from an inlet with gas withdrawn from the drying chamber, heats the mixed gases, and supplies the mixed gases to a second portion of the drying chamber.
In a preferred embodiment, the dryer is a so-called membrane dryer that substantially maintains its drying capacity under continuous use, without the need for regeneration. For use in drying particulate materials such as plastic pellets, the dryer may preferably be constructed to produce an output stream of air (or other suitable drying gas) having a dew point not exceeding 0xc2x0 F., preferably not exceeding xe2x88x9220xc2x0 F. and, more preferably, as low as at least about xe2x88x9240xc2x0 F. The invention is not restricted to the use of membrane dryers, but such dryers are advantageous from the standpoint of cost and simplicity of installation and operation. They can also achieve low dew points consistent with the preferences noted above.